Rollup merge of #40536 - kevinmehall:dedup_docs_for_dedup_by, r=steveklabnik

Fix documentation for Vec::dedup_by.

The previous docstring was copied from dedup_by_key.

src/libcollections/vec.rs

@@ -838,7 +838,11 @@ impl<T> Vec<T> {
         self.dedup_by(|a, b| key(a) == key(b))
     }
 
-    /// Removes consecutive elements in the vector that resolve to the same key.
+    /// Removes consecutive elements in the vector according to a predicate.
+    ///
+    /// The `same_bucket` function is passed references to two elements from the vector, and
+    /// returns `true` if the elements compare equal, or `false` if they do not. Only the first
+    /// of adjacent equal items is kept.
     ///
     /// If the vector is sorted, this removes all duplicates.
     ///